1. Field of the Invention
The present invention relates to a liquid crystal display and a method for fabricating the same, and more particularly, to a liquid crystal display with a simplified fabrication process having reduced number of masks and with enhanced yield and luminance due to enhanced aperture ratio and a method thereof.
2. Discussion of the Related Art
Recently, displays are becoming increasingly important as visual information transmission mediums. The key to developing the displays depends on low power consumption, a thin profile, light weight, and superior picture quality. A liquid crystal display (LCD), which is one of the major product types within the flat panel display (FPD) market, not only satisfies these needs but also enables mass production. As a result, the LCD device is replacing the cathode ray tube (CRT).
The liquid crystal display (LCD) device displays an image by supplying data signals according to image information to liquid crystal cells arranged in a matrix form thereby controlling light transmittance of the liquid crystal cells. The LCD mainly employs an active matrix driving mode in which an amorphous silicon thin film transistor (a-Si TFT) is used as a switching element to drive liquid crystals of a pixel portion.
However, a field effect mobility (˜1 cm2/Vsec) of the amorphous silicon thin film transistor imposes a limitation for use in peripheral circuits requiring a high speed operation greater than 1 MHz. Accordingly, research for simultaneously forming a pixel portion and a driving circuit portion on a glass substrate using polycrystalline silicon (poly-Si) having a field effect mobility greater than that of amorphous silicon thin film transistor is being actively performed.
The polycrystalline silicon thin film transistor has been applied to a small sized module, such as a camcorder, since a liquid crystal color television has been developed in 1982. Since the polycrystalline silicon thin film transistor has a low photosensitivity and a high field effect mobility, there are several advantages including the driving circuit being able to be directly fabricated on a substrate.
Increased mobility can increase the operation frequency of the driving circuit portion, which determines the number of driving pixels, thereby enhancing the fineness of a display device. In addition, increased mobility can decrease the charging time of the signal voltage in the pixel portion. Accordingly, the distortion of a transmitting signal can be reduced and the picture quality can be enhanced. The amorphous silicon thin film transistor has a high driving voltage of 25V, whereas the polycrystalline silicon thin film transistor has a relatively low driving voltage of 10V. Therefore, the power consumption can be decreased using a polycrystalline silicon thin film transistor.
FIG. 1 is a plane view showing a structure of a driving circuit integrated LCD device in which a driving circuit portion is integrated on an array substrate according to the related art. As shown in FIG. 1, the LCD device comprises a color filter substrate 5, an array substrate 10, and a liquid crystal layer (not shown) formed between the color filter substrate 5 and the array substrate 10.
The array substrate 10 includes a pixel portion 35 forming unit pixels that are arranged in a matrix form, and a driving circuit portion 30 including a data driving circuit 31 and a gate driving circuit 32 that are arranged at an outer periphery of the pixel portion 35. Although not shown, the pixel portion 35 of the array substrate 10 includes a plurality of gate lines and data lines arranged horizontally and vertically on the array substrate 10 to define a plurality of pixel portions. A thin film transistor (TFT) is formed at each intersection between the gate lines and the data lines and, a pixel electrode is formed at the pixel portion. Here, the TFT is a switching device that applies a voltage to the pixel electrode using a field effect transistor (FET). According to the voltages applied to the gate lines and data lines, the TFT controls a current flow through the FET by an electric field.
The driving circuit portion 30 of the array substrate 10 is positioned at an outer periphery of the pixel portion 35, as the array substrate 10 is more protruded than the color filter substrate 5. The data driving circuit portion 31 is positioned at a longer side of the array substrate 10, and the gate driving circuit portion 32 is positioned at a shorter side of the array substrate 10. The data driving circuit portion 31 and the gate driving circuit portion 32 both use TFTs having a complementary metal oxide semiconductor (CMOS) structure to serve as an inverter to properly output an input signal. The CMOS is an integrated circuit having an MOS structure used at a driving circuit portion TFT that requires a high speed signal processing, and CMOS includes both an n-channel TFT and a p-channel TFT. The speed and density characteristics of a CMOS corresponds to that of an intermediate level of an NMOS and a PMOS.
The gate driving circuit portion 32 and the data driving circuit portion 31 supply a scan signal and a data signal to the pixel electrode through the gate line and the data line, respectively, wherein the gate lines and the data lines are connected to an external signal input port (not shown). The gate driving circuit portion 32 and the data driving circuit portion 31 control an external signal input that comes through the external signal input port and output them to respective pixel electrodes.
A color filter (not shown) for producing colors and a common electrode (not shown) facing the pixel electrode formed at the array substrate 10 are formed at the pixel portion 35 of the color filter substrate 5. The color filter substrate 5 and the array substrate 10 are provided with a cell gap to be separated from each other by a spacer (not shown), and are attached to each other by a seal pattern (not shown) formed at an outer periphery of the pixel portion 35 to form a unit LCD panel. The color filter substrate 5 and the array substrate 10 are attached to each other by a bonding key formed on the color filter substrate 5 or on the array substrate 10.
Since the driving circuit integrated LCD device uses a polycrystalline silicon TFT, excellent device characteristics, such as high picture quality, enhanced fineness, and low power comsumption, can be obtained. However, the driving circuit integrated LCD device having an n-channel TFT and a p-channel TFT on the same substrate requires more complicated fabrication processes than an amorphous silicon TFT LCD having only a single type channel. In other words, to fabricate the array substrate including the polycrystalline silicon TFT, a large number of photolithography processes are required.
The photolithography processes form desired patterns on a substrate by transferring a pattern from a mask to the substrate. The photolithography processes include a plurality of processes, such as a photoresist depositing process, an exposing process, and a developing process. Accordingly, the photolithography process degrades the production yield, and thus creates a high probability of defects in the TFT. In particular, since a mask designed to form a pattern is very expensive, the fabrication cost for an LCD device increases when the number of masks increases.